Single-use sanitary paper tissue products are widely used. Such items are commercially offered in formats tailored for a variety of uses such as facial tissues, toilet tissues and kitchen roll towels. The formats, i.e. basis weight, thickness, strength, sheet size, dispensing medium, etc. of these products often differ widely, but they are linked by the common process by which they are usually produced, the so-called dry creping process. They are further linked by the common consumer desire for a pleasing tactile impression, i.e. softness.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is provided by a combination of several physical properties. One of the most important physical properties related to softness is generally considered by those skilled in the art to be the stiffness of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the strength of the web.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions.
Papermaking in general and specifically creped tissue paper manufacture is an old art. As such, it has had many years to develop to satisfy continuing consumer desires for more and more consumption at greater and greater economy. This long history has permitted papermaking machines to grow larger in size and faster in speed. The size and scale of these processes now often limit the ability of the product designer to effectively meet the before mentioned consumer need for tactile impression and product strength without sacrificing some of the gains made in machine capacity. Those skilled in the art will recognize that this is because many of the factors known to positively affect tactile impression of the product tend to be adversely affected by size and particularly speed of the papermaking and creping process.
Research and development efforts have been directed toward the improvement of softness or at least maintaining the softness quality while at the same time, continuing to increase the production capacity of papermaking units.
Creping is a means of mechanically compacting paper in the machine direction. The result is an increase in basis weight (mass per unit area) as well as dramatic changes in many physical properties, particularly when measured in the machine direction. Creping is generally accomplished with a flexible blade, a so-called doctor blade, against a Yankee dryer in an on machine operation.
In creped paper technology, reducing the percent crepe (the amount by which the paper web is foreshortened in creping) of the creped papermaking process allows the basis weight of the web traveling through the process up to the creping blade to be increased without increasing the basis weight of the final product. Papermaking generally increases in efficiency as basis weight is increased; therefore, it is desirable to use as low percent crepe as otherwise permitted by the process and the product.
A Yankee dryer is a large diameter, generally 8-20 foot drum which is designed to be pressurized with steam to provide a hot surface for completing the drying of papermaking webs at the end of the papermaking process. The paper web which is first formed on a foraminous forming carrier, such as a Fourdrinier wire, where it is freed of the copious water needed to disperse the fibrous slurry is generally transferred to a felt or fabric in a so-called press section where de-watering is continued either by mechanically compacting the paper or by some other de-watering method such as through-drying with hot air, before finally being transferred in the semi-dry condition to the surface of the Yankee for the drying to be completed.
The wet web has a natural adhesion to the Yankee dryer surface. One skilled in the art will recognize that the adhesion results primarily from the action of the water and the coating on the Yankee, the coating comprising the soluble or residual components of the papermaking composition which build up on the dryer surface with continued operation.
Often, the adhesion achieved of the semi-dry web to the Yankee arising from this natural coating is insufficient. The result is that the product is marred by areas of skipped crepe giving poor product appearance and performance and the operation is plagued by low tension in the sheet causing weaving and fluttering making it difficult to wind a wrinkle-free roll neatly enough to be utilized in subsequent operations needed to convert the product into its finished form.
Those skilled in the art will recognize that the difficulty in achieving sufficient adhesion of the traveling semi-dry web to the Yankee will be more pronounced when the moisture content of the web is low. Another factor is the fraction of the web surface which is adhered to the Yankee surface. In particular, the papermaking process for making pattern densified paper such as described by Sanford and Sisson in U.S. Pat. No. 3,301,746 issued on Jan. 31, 1967 and its progeny are particularly prone to the before-described shortfall in natural adhesion. The '746 process and its progeny create a relatively low moisture content web at transfer and only adhere a fraction of the surface of the web to the Yankee dryer.
When the natural adhesion is too low, it is sometimes supplemented with the addition of adhesion promoters. Such adhesion promoters can be added to the papermaking composition prior to reaching the Fourdrinier wire, or they can be added by spraying them on the surface of the web or on the surface of the Yankee.
One example of a supplementary adhesion promoter recommended for adding to the papermaking composition is provided by Latimer, U.S. Pat. No. 4,406,737, wherein described is a method of creping paper comprising incorporating in a paper web or paper pulp subsequently formed into a web of paper, a cationic water soluble addition polymer.
An example of a composition recommended for spraying onto the surface of the web or on the Yankee is provided by Bates, U.S. Pat. No. 3,926,716, wherein described is a process for manufacturing a soft and absorbent tissue paper web comprising applying an aqueous polyvinyl alcohol solution to cause adherence of the web to a rotating cylindrical surface.
Adhesion aids of this type are effective in raising the level of adhesion to the dryer, which can be vital to the process of making strong and soft creped tissue paper, especially of the pattern densified type. However, they do not have any appreciable positive effect in regards to allowing a reduction in percent crepe. Indeed, the improvement in adhesion often causes the tension in the web between the creping blade and the wind-up reel to be tightened making it necessary to increase the percent crepe to prevent frequent breaks due to over-tensioning when these materials are used.
Chemical debonding agents have been disclosed in various references such as U.S. Pat. No. 3,554,862, issued to Hervey et al. on Jan. 12, 1971. These materials include quaternary ammonium salts such as cocotrimethylammonium chloride, oleyltrimethylammonium chloride, di(hydrogenated)tallow dimethyl ammonium chloride and stearyltrimethyl ammonium chloride.
Shaw, in U.S. Pat. No. 3,821,068, issued Jun. 28, 1974, also teaches that chemical debonders can be used to reduce the stiffness, and thus enhance the softness, of a tissue paper web.
Emanuelsson et al., in U.S. Pat. No. 4,144,122, issued Mar. 13, 1979, teach the use of complex quaternary ammonium compounds such as bis(alkoxy(2-hydroxy)propylene) quaternary ammonium chlorides to soften webs.
Armak Company, of Chicago, Ill., in their bulletin 76-17 (1977) disclose the use of dimethyl di(hydrogenated)tallow ammonium chloride in combination with fatty acid esters of polyoxyethylene glycols to impart both softness and absorbency to tissue paper webs.
Conventional quaternary ammonium compounds such as the well known dialkyl dimethyl ammonium salts (e.g. ditallow dimethyl ammonium chloride, ditallow dimethyl ammonium methyl sulfate, di(hydrogenated)tallow dimethyl ammonium chloride etc. . . . ) are effective chemical bonding inhibiting agents.
While promoting the softness of webs, these materials are all expected to have adverse effects on the paper product, most particularly, a reduction in strength of paper webs containing them because of their tendency to inhibit the formation of fiber to fiber bonds.
Becker et al. in U.S. Pat. No. 4,158,594, issued Jan. 19, 1979, describe a method they contend will form a strong, soft, fibrous sheet. More specifically, they teach that the strength of a tissue paper web (which may have been softened by the addition of chemical debonding agents) can be enhanced by adhering, during processing, one surface of the web to a creping surface in a fine patterned arrangement by a bonding material (such as an acrylic latex rubber emulsion, a water soluble resin, or an elastomeric bonding material) which has been adhered to one surface of the web and to the creping surface in the fine patterned arrangement, and creping the web from the creping surface to form a sheet material.
The use of resins to enhance the strength of a paper web is widely known. For example, Westfelt described a number of such materials and discussed their chemistry in Cellulose Chemistry and Technology, Volume 13, at pages 813-825 (1979). Freimark et al. in U.S. Pat. No. 3,755,220 issued Aug. 28, 1973 mention that certain chemical additives known as debonding agents interfere with the natural fiber-to-fiber bonding that occurs during sheet formation in paper making processes. This reduction in bonding leads to a softer, or less harsh, sheet of paper. Freimark et al. go on to teach the use of wet strength resins in conjunction with the use of debonding agents to off-set the undesirable effects of the debonding agents.
Unfortunately, the amount of strength loss that can be offset in these ways is limited. One reason for this is because the effects of strength resins taper off rapidly in low density tissue structures as their substantivity is spent.
In addition, the addition of strength resins to counteract the deleterious effects of bonding inhibitors does not necessarily overcome other side effects of debonders on the process, most notably a decrease in adhesion to the Yankee dryer which causes the before-mentioned operational difficulties. Accordingly, the use of chemical bonding inhibitors of the beforementioned types have been relegated to relatively minor levels of incorporation and thus relatively minor influence on the product.
It is therefore an object of this invention to provide a papermaking composition capable of being converted into creped paper products that are both strong and soft.
It is a further object of this invention to provide a papermaking composition which improves the operating efficiency of the dry creping process.
These and other objects are obtained using the present invention, as will become readily apparent from a reading of the following disclosure.